The present invention refers to an acceleration threshold or acceleration limit value sensor and to a method of producing an acceleration threshold or acceleration limit value sensor.
1. Description of Background Art
According to the background art, it is known to use macroscopic mechanical arrangements for supervising acceleration and shock events, e.g. during the transport of valuable, sensitive goods. Such an arrangement comprises a transparent plastic container with a metal ball held by four springs at a position of rest. When a mechanical acceleration or a shock load above a limit value occurs, the ball leaves its position of rest. Subsequently, it can be found out by optical inspection whether such a known limit value sensor has been subjected to an acceleration or shock load above a limit value. Such macroscopic mechanical arrangements are complicated. In addition, such known arrangements can only be read optically, i.e. by observation.
In the field of technology, numerous applications of micromechanical structures for detecting accelerations, speeds and forces are additionally known. By means of such micromechanical structures, e.g. acceleration detections can be carried out by means of capacitive measurements as well as other measurement principles, e.g. by closing contacts. Such micromechanical structures are, however, not able to store information on the detected accelerations mechanically and therefore without auxiliary power.
2. Description of Prior Art
DE 748 408 C shows a maximum acceleration meter in the case of which a maximum acceleration is related to a permanent deformation or to the breaking of a structural material. For this purpose, rods, which are formed of a brittle or permanently deformable structural material, are connected to a base plate on one side thereof, whereas the non-fixed ends of the rods have masses attached thereto. The cross-section of such a rod having a mass attached thereto, which is subjected to the highest stress, and, consequently, the predetermined breaking point is the respective point where the rod is fixed in the base plate. It follows that, a maximum acceleration to be determined can be detected according to DE 748 408 C on the basis of a breaking of said rods.
WO-A-9111722 describes a semiconductor acceleration sensor consisting of a fastening section, an etched silicon spring and a mass attached to said spring. A resistance loop is provided over the length of the spring, said resistance loop constituting a spring break indicator. This spring break indicator serves to indicate the readiness for operation of the acceleration sensor disclosed in WO-A-9111722 or a damaged condition of said acceleration sensor. WO-A-9111722 does not disclose an acceleration threshold sensor in which the breaking of a predetermined breaking point is intended to indicate the occurrence of an acceleration exceeding a predetermined acceleration.
It is the object of the present invention to provide an acceleration threshold sensor which has a simple structural design, which can be produced at a reasonable price and is always ready to carry out a measurement, and which is also able to store, without any auxiliary power, that the sensor has been subjected to an acceleration exceeding a predetermined acceleration.
This object is achieved by an acceleration threshold sensor sensor comprising a supporting device; a seismic mass; and a connecting device with the aid of which the seismic mass is attached to the supporting device, said connecting device being provided with a predetermined breaking point interrupting the connection between the seismic mass and the supporting device when said seismic mass is subjected to an acceleration exceeding a predetermined acceleration, wherein the supporting device, the seismic mass and the connecting device are formed integrally from a silicon layer by means of a micromechanical method in such a way that the silicon layer is provided with an opening extending therethrough and having arranged therein the seismic mass, the border of said opening defining the supporting device, the seismic mass being connected on a first side thereof to the neighbouring side of the supporting device by a first bar-shaped connection piece which defines the predetermined breaking point, and said seismic mass being connected to the side of the supporting device located opposite said neighbouring side by means of a second and a third connection piece, said second and third connection pieces being connected to the seismic mass in the area of the first side of said seismic mass and extending symmetrically on both sides of the seismic mass to the opposite side of the supporting device, said second and third connection pieces being implemented such that they do not substantially influence the mechanical properties of the seismic mass.
In the case of the acceleration threshold sensor according to the present invention, the support means, the seismic mass and the connection means are formed integrally from a semiconductor layer by means of a micromechanical method in such a way that the predetermined breaking point of the connection means is formed by a configuration of the semiconductor layer. Such an acceleration threshold sensor has a simple structural design, it can be produced at a reasonable price, it is always ready to carry out a measurement and it does not, in principle, require any auxiliary power for functioning. In special circumstances, the sensor according to the present invention can be used in an advantageous manner in battery-operated systems in long-term operation, where it will consume a minimum amount of energy, and it can be read electrically at an arbitrary time.
The micromechanical acceleration threshold sensor is provided with breaking structures consisting of a seismic mass in the form of a board or a similar structure and of a substrate and bars which are secured to the mass. These breaking structures can define a conductor loop which is interrupted when the bars break. Hence, the threshold sensors according to the present invention can be read electrically or also optically, e.g. by visual inspection. Such breaking structures can be used as sensors for limit accelerations of all kind, as detection means for specific stress events of devices and, when several such breaking structures are arranged in the form of an array in a system, they can be used as digital acceleration sensors.
A further object of the present invention is to provide a method of producing an acceleration threshold sensor.
This object is achieved by a method of producing an acceleration threshold sensor including a supporting device, a seismic mass, and a connecting device with the aid of which the seismic mass is attached to the supporting device, said connecting device being provided with a predetermined breaking point interrupting the connection between the seismic mass and the supporting device when said seismic mass is subjected to an acceleration exceeding a predetermined acceleration, said method comprising the following steps or acts: providing a SIMOX substrate or another SOI substrate or a silicon substrate provided with a silicon oxide layer or a silicon start layer; depositing a silicon epitaxial layer or a silicon CVD layer on a thin silicon film of the SIMOX substrate or on an oxide layer of the SOI substrate or on the silicon start layer; producing a conducting-path system and a contacting system on the silicon layer deposited in the previous step; producing and structuring a passivation layer over the silicon layer deposited in the second step and the conducting-path system and the contacting system, with the property that this layer acts as an etching mask during a subsequent trench-etching process; producing a back mask and carrying out a back-etching process, an oxide layer of the SIMOX substrate, an oxide layer of the SOI substrate or an oxide layer provided on the silicon start layer acting as an etch stop; carrying out etching from the back of the structure for removing the oxide layer; applying a protective layer to and structuring same on the front of the structure and applying a protective layer to the back of the structure; carrying out a subsequent trench-etching process so as to form and expose the seismic mass and the connection means of the acceleration threshold sensor; and removing the protective layers.